"I was training like an animal," he said, working out seven days a week. But he seemed to have reached his biological limit: 5 feet 8 inches tall, 150 pounds.

"Finally, it dawned on me," he said. Those huge men at his gym who insisted they were simply lifting weights were dissembling. "There was something they were not telling me," Keogan said.

Thus Keogan, a 30-year-old salesman who lives near Boston, entered the world of anabolic steroids -- testosterone and other drugs that act like it, which can build muscle, fast.

He soon was taking 4,000 milligrams of testosterone a week, which he bought from dealers at his gym. (A man his age normally produces about 35 milligrams a week.)

Within 20 weeks, he weighed 200 pounds. He was massive. People stared, he said; crowds parted. Acquaintances no longer recognized him.

"People would look at me," he said, and ask, "'What did you do?'"

Now, as more and more men, and some women, are seeking large, chiseled bodies, more are learning the bitter secret of that look: It almost always takes some chemical assistance, from drugs that are often illegal but are readily available.

Anabolic steroids are nothing like cortisone creams for itchy skin or corticosteroid injections to reduce inflammation. These are drugs that build muscle.

Recently, police officers in a prosperous community in Utah arrested three high school boys who were smuggling the drugs from Mexico, planning to sell them to classmates.

But the use of steroid drugs in high schools is just part of a larger pattern. Use has spread from weightlifters to bodybuilders to elite athletes to high school and college athletes, as well as to groups, such as gay men and gym aficionados, who simply want to improve their appearance.

It is hard to quantify their use -- federal surveys of adults' drug use do not ask. There is a huge underground market for anabolic steroids, including ones approved only for horses, and steady traffic by Americans in Mexico, where, as in many other countries, they are sold over the counter.

Some people get prescriptions from doctors who overlook the fact that the only legal use of testosterone in the United States is to treat a real medical deficiency.

Added to that is an enormous market in nutritional supplements, which are very loosely regulated and whose makers say they have the same effects as testosterone.

Prescriptions for anabolic steroids have soared in recent years, to 1.5 million in 2001 from 806,000 in 1997, according to IMS Health, a company that monitors drug sales.

But it is impossible to know how much is being taken for legitimate medical needs.

By all accounts, a small minority of Americans use the drugs, but medical experts are concerned. There are suspicions -- but very little solid evidence -- that anabolic steroids and drugs that act like them can lead to serious long-term effects, including heart attacks, strokes and cancer.

There are stories that the drugs can sometimes turn placid people violent. There are concerns that in some sports, those who want to compete have little choice but to take them.

The personality change was what worried Keogan -- he quit after jumping out of his car to argue with another driver in a fit of rage, leaving his car to drift away.

Now his body has shrunk to its former size, and he struggles to lift weights that were once a warm-up to his real lifting.

Dr. Shalender Bhasin, a professor of medicine at UCLA, says it should not be surprising that people keep taking the drugs.

They do build muscle, decrease fat and improve athletic performance. In the meantime, society is sending mixed messages: No one should take steroids, but athletes should be winners and everyone should strive to be strong and muscular.

"There is some degree of denial and hypocrisy with the use of these compounds," Bhasin said. "We discourage it by punishing a few people now and then to show our displeasure, but we tolerate their use. We haven't taken a stand as a society, and the widespread use of these agents is reflective of our ambivalence."

Anabolic steroids came to weightlifting in the 1950s, when Russians began taking testosterone and winning championships, said John D. Fair, a competitive weightlifter and historian of weightlifting at Georgia College and State University in Milledgeville.

By 1960, a few American weightlifters had begun secretly taking them too. By 1970, other elite athletes had discovered anabolic steroids.

Drugs came later to women's sports. In East Germany and other Eastern bloc countries, female swimmers and other athletes were secretly taking steroids in the 1960s, but almost no female athletes elsewhere were using them.

Jan Todd of the University of Texas at Austin became a competitive lifter in the 1970s. By the 1980s, many female weightlifters had begun taking steroids.

Todd, who refused, saw her lifting career come to an end.

As steroid use seeped into sports and bodybuilding, historians say, muscles became more desirable for ordinary men and women.

Men, Fair said, became entranced with Arnold Schwarzenegger and began craving big, hard bodies.

Women, who had been barred from weight-lifting rooms, found that the doors had opened with the passage of Title IX, the 1973 law that required universities receiving federal funds to provide women with equal access to athletic facilities.

Now, said Dr. Harrison Pope Jr., a Harvard Medical School psychiatrist who studies the culture of bodybuilding, the transformation is everywhere. Women in the centerfolds of men's magazines are increasingly muscular. Male models in advertisements and on magazine covers increasingly appear shirtless, muscles bulging.

For athletes, the mixed messages are all too clear.

Sports fans do not want to see smaller, slower linemen and backs in football, or snooze through the 150th-fastest running of the 100 meters at the next Olympics, said Dr. Charles E. Yesalis, a professor of health and human development at Pennsylvania State University and the author of the book "Anabolic Steroids in Sports and Exercise."

Some anabolic steroids can lower levels of HDL cholesterol, the type that normally protects against heart disease. A lower HDL level may lead to an increased risk of heart attack and stroke.

They may also fuel the growth of some cancers, particularly those of the prostate. They suppress the body's own sex hormone production, which can cause infertility in people who take the drugs.

But no one has done a long-term study of people who took huge doses of anabolic drugs, and for the most part, no one knows what medical problems the drugs may cause or how likely they are.

Even less is known about the nutritional supplements that are advertised as mimicking the effects of drugs such as testosterone.

Only now is a study being conducted to see if any of these supplements increase muscle size. It is a study of guinea pigs.